A.C voltmeters using rectifier

Transcription

1 Lecture 5 A.C voltmeters using rectifier The PMMC movement used in d.c. voltmeters can be effectively used in a.c. voltmeters. The rectifier is used to convert a.c. voltage to be measured, to d.c. This d.c., if required is amplified and then given to the PMMC movement. The PMMC movement gives the deflection proportional to the quantity to be measured. The r.m.s. value of an alternating quantity is given by that steady current (d.c.) which when flowing through a given circuit for a given time produces the same amount of heat as produced by the alternating current which when flowing through the same circuit for the same time. The r.m.s value is calculated by measuring the quantity at equal intervals for one complete cycle. Then squaring each quantity, the average of squared v,llues is obtained. The square root of this average value is the r.m.s. value. The r.m.s means root-mean square i.e. squaring, finding the mean i.e. average and finally root. If the waveform is continuous then instead of squaring and calculating mean, the integratioll is used. Mathematically the r.m.s. value of the continuous a.c. voltage having time period T is given by, The 1/T term indicates mean value or average value.

2 If the a.c. quantity is continuous then average value can be expressed mathematically using an integration as, The form factor is the ratio of r.m.s. value to the average value of an alternating quantity. Basic rectifier type voltmeter: Fig1.6: Basic Rectifier type voltmeter The diodes D1 and D2 are used for the rectifier circuit. The diodes show the nonlinear behaviour

3 for the low currents hence to increase the current through diode D1, the meter is shunted with a resistance Rsh' This ensures high current through diode and its linear behaviour. When the a.c. input is applied, for the positive half cycle, the diode 01 conducts and causes the meter deflection proportional to the average value of that half cycle. In the negative cycle, the diode O2 conducts and 01 is reverse biased. The current through the meter is in opposite direction and hence meter movement is bypassed. Thus due to diodes, the rectifying action produces pulsating d.c. and lile meter indicates the average value of the input. A.C voltmeter using fullwave rectifier: The a.c. voltmeter using full wave rectifier is achieved by using bridge rectifier consisting of four diodes, as shown in the Fig 1.7. Fig 1.7: A.C voltmeter using fullwave rectifier Electronic multimeter: For the measurement of d.c. as well as a.c. voltage and current, resistance, an electronic multimeter is commonly used. It is also known as Voltage-Ohm Meter (VOM) or m ultimeter The important salient features of YOM are as listed below. 1) The basic circuit of YOM includes balanced bridge d.c. amplifier.

4 2) To limit the magnitude of the input signal, RANGE switch is provided. By properly adjusting input attenuator input signal can be limited. 3) It also includes rectifier section which converts a.c. input signal to the d.c. voltage. 4) It facilitates resistance measurement with the help of internal battery and additional circuitry. 5) The various parameters measurement is possible by selecting required function using FUNCTION switch. 6) The measurement of various parameters is indicated with the help of indicating Meter. Use of multimeter for D.C measurement: Fig 1.8:Use of multimeter for dc measurement. For getting different ranges of voltages, different series resistances are connected in series which can be put in the circuit with the range selector switch. We can get different ranges to measure the d.c. voltages by selecting the proper resistance in series with the basic meter. Use of multimeter as ammeter: To get different current ranges, different shunts are connected across the meter with the help of range selector switch. The working is same as that of PMMC ammeter

5 Fig 1.9: Use of multimeter as ammeter Use of multimeter for measurement of A.C voltage: Fig:1.10: Multimeter for measurement of AC.Voltage The rectifier used in the circuit rectifies a.c. voltage into d.c. voltage for measurement of a.c. voltage before current passes through the meter. The other diode is used for the protection purpose. Use of multimeter for resistance measurement: The Fig shows ohmmeter section of multimeter for a scale multiplication of 1. Before any measurement is made, the instrument is short circuited and "zero adjust" control is varied until the meter reads zero resistance i.e. it shows full scale current. Now the circuit takes the form of a variation of the shunt type ohmmeter. Scale multiplications of 100 and 10,000 can

6 also be used for measuring high resistances. Voltages are applied the circuit with the help of battery. Fig1.11:Multimeter for resistance measurement.